Method and apparatus for reviewing defects by detecting images having voltage contrast

ABSTRACT

In a traditional method for automatically obtaining high-magnification images of defects by using an electron microscope for defect-reviewing of a semiconductor wafer, high-magnification images of a voltage contrast changing part are obtained in the case of defects generating voltage contrast change, this made difficult to observe defects themselves generating voltage contrast change. In the present invention, based on energy of secondary electron to be detected, after obtaining two types of images, namely an image making voltage contrast conspicuous easily, and an image not making it easily, and acquiring a shape change area adjacent to a voltage contrast change area based on this area as a defect location, a high-magnification image can automatically be obtained.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese applicationJP2006-081882 filed on Mar. 24, 2006, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

This invention relates to a scanning electron microscope (hereinafterreferred to as SEM) to obtain images of an observation object, byirradiating a focused electron beam to a semiconductor device in courseof manufacture at a front-end semiconductor device process, anddetecting electrons released from the irradiating location, and inparticular, relates to a SEM type semiconductor wafer inspectionapparatus required to obtain high-magnification images, a review SEM toobserve defects detected in a semiconductor wafer, in more detail, andfurther a method for reviewing, in high-magnification, defects havingvoltage contrast at an inspection apparatus detecting the defects and anapparatus thereof.

With miniaturization of semiconductor devices, it has becomeincreasingly difficult to control a front-end semiconductor devicemanufacturing process, and it has been important to inspect and reviewafter a circuit is formed. As known examples concerning generalinspection and review, an example disclosed in JP-A-10-135288 iswell-known, wherein such a sequence is disclosed: inputting, by a reviewunit, defects coordinates detected by an inspection apparatus usingmainly an optical microscope; obtaining low-magnification images aftertransferring the field of view of a SEM type review apparatus into thecoordinates; and after localizing a defect location from the imageobtained, obtaining high-magnification images at the defect location.However, by a review, which is also shown in this known example,conducted after inspection by using an optical microscope, withmulti-stratification of a semiconductor device pattern, and, further,due to the increase in aspect ratio by finer patterning trend, it hasbecome difficult to observe defects generated at the bottom havingtrench configuration between patterns, by an optical microscope.

A problem of high aspect ratio is more serious concerning defectsgenerated at the bottom of a hole, and it is almost impossible to detectby an optical inspection apparatus. Therefore, though an inspectionbased on images obtained by a SEM has prevailed to detect these defects,imaging by a SEM has a problem that time required for imaging by a SEMis generally longer than that by an optical microscope; this disables toinspect with high throughput. To solve this problem, recently a methodhas been conducted, wherein a test pattern different from an actualcircuit of a semiconductor device is formed and only this test patternis inspected.

As a well-known example of this method, for example, as disclosed in US2004/0207414, an inspection method is known, wherein, a test pattern, inwhich voltage contrast (potential contrast) is easily changed, is formedwhen electrical defects are generated in a semiconductor device, andafter detecting first a pattern generating voltage contrast, only thepattern generating voltage contrast is inspected; this enables toinspect relatively at high speed even by a SEM type inspectionapparatus.

Further, as a method for distinguishing defects generating voltagecontrast from shape defects, as disclosed in U.S. Pat. No. 6,642,726,there is a method, wherein, in the case where a defect size is large andthe ratio of a short side to a long side of a rectangular area in anarea detected, is large, the defects are classified as ones generating avoltage contrast phenomenon, and it is judged that the defect locationis a position where brightness changes in the case of open defects, anda defect area extends across patterns in the case of short defects.

Further, as a method for finding a defect location, though there is nodescription on voltage contrast change, for example, as described inJP-A-2003-098114, a method is disclosed, wherein, for an image ofdefects, parts where the same pattern is imaged at different positions,are searched by every local region; and a reference image is composedbased on the pattern searched; and a defect location is detected bycomparing the defect image with a comparison image composed; thisenables to calculate a defect location without imaging a referenceimage.

Among conventional methods described above, first in the review methoddisclosed in JP-A-10-135288, it had a problem of difficulty in obtaininga high-magnification image of defects. Generally, with fatal defectsgenerated on a test pattern, voltage contrast change is caused due toelectrical characteristic change generated by the defects. In thewell-known example, because a high-magnification image is obtained bycomparing a defect image with a reference image, and then the differenceis detected as defects, the central part of a voltage contrast changearea, which is imaged relatively large, is imaged as a defect location.In reviewing defects, however, because what is required is, notconfirming that voltage contrast change is generated, but reviewingdefects themselves causing the voltage contrast, generally, this methodcannot satisfy user's requirement. In addition, in the case where ahigh-magnification image is obtained only based on defect coordinatesoutput from an inspection apparatus, because stages for moving a waferare basically different between those in an inspection apparatus and ina review apparatus; this leads to insufficient correspondence betweencoordinates, and it becomes difficult to obtain a defect image, becausedefects are not located within a small field of view, which isindispensable in the case of trying to inspect defects athigh-magnification.

Further, U.S. Pat. No. 6,642,726 describes, as a method for findingdefects on a test pattern in a SEM type inspection apparatus, methodsfor finding a defect location from defects with voltage contrast by aSEM, including a method for distinguishing voltage contrast defects froma physical defects; and a method for identifying a defect location ofdefects causing voltage contrast, however, these methods have a problemof inability to stably detect short defects. A SEM type review apparatusis required to review defects with voltage contrast, however, anenlarged field of view of a review apparatus is required to observedefects detected by a SEM type inspection apparatus, by a reviewapparatus, regardless of alignment error between a SEM type inspectionapparatus and a review apparatus. Meanwhile, in the well-known example,it is described that defects which cause short defects are extracted asdefects which connect a pattern having voltage contrast differencebetween a defect image and comparison image thereof, and a patternadjacent to the pattern, however, in the case of imaging at a wide fieldof view, a distance between patterns is imaged as quite short. In thecase where voltage contrast is generated, it is common that defects areimaged as bright as a pattern and it is difficult to identify alocation, in particular, in the case of microscopic defects presentbetween patterns.

Further, as the second problem, in the case where difference of voltagecontrast is generated, and in the case where a reference image iscomposed by using the a method disclosed in JP-A-2003-098114, becausedifference by voltage contrast is imaged comparatively large, a problemarises of a phenomenon that brightness change remains due to voltagecontrast generated by a defect, occurs in a reference image; this makesit impossible to detect the whole of voltage contrast abnormal parts. Inparticular, in voltage contrast generated from open defects, thisbecomes a problem to find a defect location. In open defects, asdescribed in U.S. Pat. No. 6,642,726, it is important to find a startingpoint where voltage contrast difference starts, however, when areference image is poorly composed, the difference area which isnormally one area, is detected as separated ones; this disables tostably find a starting point of voltage contrast.

SUMMARY OF THE INVENTION

The present invention provides a method and an apparatus thereof tostably review defects such as short defects and open defects withvoltage contrast, detected by other apparatuses.

That is, the present invention is accomplished by inputting a defectlocation obtained by an inspection of an observation object; obtaining aSEM image so as to have defects with voltage contrast detected by otherinspection apparatus (for example, a SEM type defect inspectionapparatus), located in a field of view; comparing the image obtained,with an image of a normal part, identifying a defect location fromdifference generated by any of defects or voltage contrast change causedby defects, or by both thereof, and obtaining a higher-magnificationimage at the defect location obtained. Further, it is possible to solvea problem to stabilize defect location accuracy, by imaging both images;namely, an image with voltage contrast easily made obvious and an imagewithout, as these images.

Additionally, in the present invention, a reference image whichcorresponds to an image of a normal part used for calculating thedifference is created from an image of a field of view including adefect obtained by a SEM imaging.

The present invention enables to review defects themselves, whichgenerate voltage contrast, including short defects caused by microscopicdefects generated between patterns. Further, it becomes possible tostably identify a starting position of a voltage contrast abnormal partcaused by open defects.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an embodiment of a basic configuration of a system forreviewing defects, furnished with a SEM.

FIG. 2 is a flowchart illustrating a sequence for obtaining images ofdefects automatically.

FIG. 3A is a drawing showing an example of a test pattern area whereopen defects are generated.

FIG. 3B is a defect image accompanied with a voltage contrast image(open) obtained by imaging the sample shown in FIG. 3A by a SEM.

FIG. 3C is a difference image of voltage contrast obtained by comparingthe defect image in FIG. 3B, with a reference image having no defects.

FIG. 3D is a drawing showing projection length of the difference areaobtained by grouping the difference area on the difference image ofvoltage contrast (open) in FIG. 3C.

FIG. 4A is a drawing showing a test pattern suitable to detect shortdefects.

FIG. 4B is a difference image obtained by comparing the image obtainedby imaging FIG. 4A by a SEM, with a reference image.

FIG. 5 is a flowchart illustrating one example of a detecting flow fordetecting defects by classifying between open defects and short ones.

FIG. 6 is other embodiment of a system configuration for reviewingdefects, furnished with a SEM, and a drawing showing a configuration,wherein SEM images are simultaneously obtained by two detectors havingdifferent energy detecting ranges.

FIG. 7A is a SEM image showing a state having defects present on a testpattern.

FIG. 7B is a reference image corresponding to the test pattern in FIG.7A.

FIG. 7C is a difference image between the defect image in FIG. 7A andthe reference image in FIG. 7B.

FIG. 7D is a drawing showing a difference image between the defect imagein FIG. 7A and the reference image composed of the defect image in FIG.7A.

FIG. 8A is a SEM image of a pattern having open defects.

FIG. 8B is a drawing showing a difference image obtained by comparingthe defect image in FIG. 8A with a reference image.

FIG. 9 is a cross-sectional drawing of a test pattern of contact chain.

FIG. 10 is a flowchart illustrating a procedure for distinguishingdefects, which cannot be observed, from those observed by a SEM.

FIG. 11 is a drawing showing one embodiment of a configuration of areview observation apparatus having both a SEM observation opticalsystem and a FIB optical system.

FIG. 12 is a drawing showing one example of a system configuration foridentifying the cause of defects.

DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described by referring toFIGS. 1 to 12. On a semiconductor wafer, patterns are formedmultilayer-structure-wise through a lot of steps. During the steps forproducing the multilayer-configuration, to monitor the productionprocess, a dimension measurement and an appearance inspection of apattern formed by each layer, and reviewing of defects detected by theappearance inspection are implemented.

Because the recent semiconductor device process has become increasinglyfiner, for imaging to make up for this, a SEM has been applied, wherebyimaging with higher resolution can be obtained than imaging by anoptical microscope. As a SEM used for this purpose, a review SEM haswidely been used. A review SEM has a major function to image defects byan SEM, after transferring a field of view into a defect location basedon defect coordinates detected by appearance inspection. FIG. 1 shows abasic configuration in the case where the present invention is appliedto a review SEM.

101 is an electron beam source, which irradiates an electron beam. Afterthe electron beam irradiated goes through condenser lenses 102 and 103,astigmatism or misalignment is corrected by an electron beam axisadjuster 104. After the electron beam is deflected by scanning units 105and 106, and the location of emitting the electron beam is controlled,the electron beam is converged by an object lens 107 and emitted to animaging object 109 of a wafer 108; as a result, a secondary electron anda backscattered electron are released from the imaging object 109, andstrike on a reflecting board 121 having a primary electron beamthrough-hole, and a secondary electron generated at this location aredeflected by ExB 110 and detected by an electron detector 111. Thesecondary electron and the backscattered electron detected by 111 areconverted to digital signals by an A/D converter 112 and stored in amemory 113. 114 is an X-Y stage, which transfers 108 and enables imagingat arbitrary position of a wafer.

115 is an image processing unit, wherein a defect location is detectedfrom an image stored in a memory 113. As this detecting method, such amethod is used, that a location where difference is present, is detectedas defects by comparing an image of a defect location, with an image ofreference location on which the same pattern with the image of a defectlocation is expected to be formed. 116 is a secondary memory apparatusenabling to memorize images stored in 113. 117 is a computer terminal,which enables to display images stored in 116 or 113. Further, inputtingdata to the terminal 117 enables a user to set various operations of theapparatus. 118 is a total control system, which controls axis adjustmentof an electron beam, deflection of an electron beam by a scanning unitand movement of a field of view by transferring the XY-stage movement.119 is an electrode for generating an electric field, and enables tocontrol a surface electric field of a wafer 108 by the total controlsystem 118.

120 is a recipe-server, wherein a data-file is stored, in which defectcoordinates output from an inspection apparatus performing inspectionsof a wafer, and the data-file is transferred to the total control system118, whereby imaging sequence for obtaining a defect image iscontrolled. In addition, it is also possible to store in therecipe-server 120, images stored in 116 and the results transferredafter processing of the images. 122 is an energy filter, by whichelectric field is locally generated so that electrons emitted from thewafer 108 can not pass through, in the case where their energy level islow.

In an apparatus with a configuration in FIG. 1, a sequence to obtain adefect image is shown in FIG. 2. First, at 201, coordinate data ondefects to be reviewed are input and then, at the step 202, defects tobe reviewed are selected. At the step 203 the XY-stage 114 istransferred and then a reference location corresponding to defectsselected at 203 are brought at the field of view of a SEM. At the step204, a reference image is obtained and then, at the step 205, theXY-stage 114 is transferred and the defects selected at the step 206 arelocated within the field of view of the SEM. At the step 207, an imageis obtained, and at the step 208, a defect location is identified bycomparing the image obtained at 204 with image at obtained at 207. Atthe step 208, AF is implemented, and at the step 209, after transferringthe field of view into the defect location identified at 207, ahigh-magnification image is obtained and the image is stored in 116.After storing the image in 116, in the case where other defects to bereviewed are present, step 202 is again performed.

Next, defects to which the present review method is applied are shown inFIGS. 3A to 3C. FIG. 3A shows an example of a test pattern area 300where a defect 302 occurs. In this example, as the defect 302 is onecausing an electrical fault, for example, in the case where open 302 isgenerated on a pattern 301 formed in the area 300 where a large patternhaving high capacitance like the test pattern is formed, the areairradiated by an electron beam charges positively when an electron beamis irradiated on this area, because the number of secondary electronsreleased is larger than that of electrons irradiated under the generalSEM imaging condition. In this situation, because the pattern isconnected with a high capacitance area in an area 303 and, becauseelectrons are supplied from the high capacitance area, a degree of thecharging is small, even though the area irradiated by electrons chargespositively.

In contrast, because a secondary electron is not supplied to an area304, a degree of positive-charging is small. In the case where anelectron is irradiated on a highly positively charged area, because asecondary electron released from the area is returned to the pattern, adark image is obtained; on the other hand, in a lowly charged area,because secondary electron is not returned, a bright image is obtained.That is, when a sample shown in FIG. 3A is imaged by a SEM, a voltagecontrast image 305 having lightness contrast as in FIG. 3B is obtained.In the case where an open defect occurs like this, the defect forms aboundary and the lightness of the patterns differs. By comparing thedefect image (equivalent to the voltage contrast image 305 in FIG. 3B)obtained at the step 206 explained in the flow in FIG. 2, with thereference image obtained at the step 204, a reference image 306 ofvoltage contrast is calculated as shown in FIG. 3C.

In the conventional review SEM, because high-magnification images areobtained with a central focus on the difference area of the differenceimage 306, though the voltage contrast changing part caused by thedefect can be imaged, the defect itself cannot be imaged. The presentembodiment makes it possible to obtain a high-magnification image with acentral focus on the defect location. Identification of an open defectoccurrence location in a test pattern essentially requires to identify adefect location by obtaining a starting point of a difference area ofvoltage contrast, after finding directionality to which a test patternis connected. In case of reviewing a defect by a SEM, thoughdirectionality of a test pattern can be decided based on design data, asit is not always possible to get design data easily in a review at anactual mass-production line, it is quite vital to obtain directionalityof a test pattern from an image based on low-magnification defect imageincluding aspect of difference area of voltage contrast.

For example, in the case of contact chain like FIG. 3A, by grouping thedifference area on the difference image 306 of voltage contrast as shownin FIG. 3D, and by obtaining Lx and Ly by calculation of projectionlength of the grouping area 308 in X and Y directions, the longer one (Yin the case of FIG. 3D) can be determined as a pattern direction. Next,by tracing the difference area 308, which is located close to the centerof a field of view, in the pattern direction, that is, in an X directionorthogonal to a Y direction, to this pattern direction, that is the Ydirection; and by finding a boundary of an area having difference fromsignal change on the difference image 306 in the difference area 308,the generating location of an open defect can be identified. Asdescribed above, even in the case where voltage contrast change occursin a wide area, both in a defect image and a reference image, defectlocation can be identified. A direction, in which voltage contrastoccurs, is obtained here based on images, however, obviously it is alsoacceptable to use CAD data or to provide inspection data withinformation concerning the direction in which voltage contrast changeoccurs.

In FIG. 4A, a structure of a test pattern suitable to detect a shortdefect is shown. 401 and 402 are conductive patterns, which areconnected with a pattern with high capacitance. In contrast, 403 and 404are patterns of floating. In 401 and 402, even when a secondary electronis released with irradiation of an electron beam, an image obtained isbright, because electrons are supplied from an area having highcapacitance. However, in 403, an image is dark, electrons are notsupplied. 405 is a defect generated across areas with these twodifferent types of patterns. As electrons are supplied to the pattern404 from this defect 405, the pattern which should be imaged essentiallydark due to floating, is imaged bright. As a difference image 406obtained from the difference between the FIG. 4A, as a defect image, anda reference image (not shown), becomes as shown in FIG. 4B, it isimpossible to stably identify a defect location in the case based onthis image.

As a result of studying the above problem, it was found that, in thiscase, it is impossible to stably detect a defect location unless imagingis performed so that difference of voltage contrast does not becomeobvious. The main reason for this is that when low-magnification SEMimage is obtained in a field of view including a base part adjacent to apattern with high capacitance, it becomes difficult to find a shortdefect across patterns from the image, because the image is obtained insimilar brightness as in a pattern having high capacitance. One methodto solve this problem includes the following method, wherein imaging isperformed by detecting a secondary electron and a backscattered electronhaving high-energy among the secondary electron and the backscatteredelectron released from an observation object; this can be realized, forexample, in the configuration shown in FIG. 1, by cutting a secondaryelectron having low energy by using an energy filter 121.

In general, a generating mechanism of a difference area of voltagecontrast is that charged electric potential of a target object rises dueto irradiation of a primary electron beam, and secondary electronshaving low energy generated at the observation object, returns to theobservation object again. Generally, because charged potential is oftenlower than or equal to 20 V (however, this value depends on the size ofa field of view), secondary electrons higher than or equal to 20 V donot return to an observation object and change of voltage contrastcaused by defects, does not practically occur. Therefore, when thepotential of an energy filter 121 is controlled so that only electronshaving energy higher than or equal to 20 V are detected, a defectlocation can be identified without influenced by voltage contrast,because an image having no voltage contrast can be obtained.

In some cases, however, because an open defect can not be identified inan image without generating voltage contrast change, after two types ofimages are obtained with and without energy-cutting by an energy filter,a defect location can stably be calculated from an image obtained, forexample, in the state of not performing energy-cutting for an opendefect, and in the state of performing energy-cutting for a shortdefect. Further, as for an open defect and a short defect, by judgingwhether voltage contrast change becomes brighter or darker than adifference image obtained from an image without energy-cutting, they canbe decided as follows: in the case of becoming darker, a defect is anopen defect, and in the case of becoming brighter, it is a short defect.

Note that an electric field to be generated by an energy filter requiressetting from an observation object, in consideration of potentialdifference between an observation object and an energy filter. Forexample, in the case where electric potential at an observation objectis −1200 V and electric potential at the position of an energy filter is0 V, because electrons having an energy of 20 eV at the point of releasefrom the observation object, come to have energy of 1,220 eV, when anelectric potential of 1,220 V is generated inside the energy filter,electrons having an energy of lower than or equal to 20 V, is cut off atthe point of release from the object.

A flow in this case is almost the same as that explained in FIG. 2,however, it is acceptable that each of images obtained at the steps 204and 206 by two cases; namely one case with application of electricpotential to the energy filter 121, and the other case withoutapplication. An algorithm flow is shown in FIG. 5. At defect imagingstep 501, in both states that the energy filter 121 is working (ON), andis not working (OFF), both defect images are obtained and SEM defectimages (defect image) are obtained; subsequently at reference patternimaging step 502, in the both states that the energy filter 121 isworking (ON), and is not working (OFF), reference pattern images areobtained and SEM reference pattern images (reference image) areobtained. Next, at defect/reference difference calculation step 503, adifference image is calculated from a defect image and a reference imageobtained in the state that the energy filter 121 is not working (OFF),and at difference digitization step 504, an area having the largerdifference, is extracted and binarized, and then at the open/shortdecision step 505, by judging whether the area extracted, is plus orminus, it is judged to be an open defect in the case of minus, and to bea short defect in the case of plus.

The pattern directionality decision step 506 is a process performed inthe case of an open defect and the pattern directionality is decidedbased on a project length of a difference area by using the method suchas explained in FIG. 3D. At the starting point calculation/defectlocation identification step 507, based on an image binarized at thedifference digitization step 504, by searching a starting point in ofthe binarized image the pattern directionality decided at the patterndirectionality decision step 506, the starting point is identified as anopen defect location.

Next, in the case where it is concluded to be a short defect at theopen/short decision step 505, the following step is the differencecalculation step 508, wherein a difference image is calculated between adefect image and a reference image, the former being obtained in thestate that the energy filter 121 is working (ON) by applying electricpotential to the energy filter 121 at the steps 501 and 502; then at thebinarizing processing/defect location identification step 509, an areawhich has difference at the difference digitization step 504, isexpanded as a defect extraction area; and in that area, by binarizingthe difference image calculated at the difference calculation step 508,a short defect location is identified.

A fault of the method for obtaining two types of images; namely in thecase of cutting a low energy electron and in the case of not cutting, bythe energy filter 121, is a decline of throughput caused by doubling thenumber of images. A method for avoiding this fault includes a system,wherein images are obtained simultaneously by not less than twodetectors having different energy detecting ranges. FIG. 6 shows a basichardware configuration in this case. The major difference fromconfiguration explained in FIG. 1, is that two types of detectors areprovided; namely one for detecting secondary low energy electrons andhigh energy electrons released in the normal direction of a wafer of anobservation object, and the other for detecting high energy electronsreleased in the low-angle elevation direction.

101 to 121 in FIG. 6 have the same functions as those in FIG. 1. 601 isa reflecting board, wherein a hole is provided in the center forelectrons released from an observation object to pass through. 602 and603 are electron detectors. Electric signals output from 602 and 603,are converted into digital signals by A/D converters 604 and 605,respectively and then stored in a memory 113. Electrons are released inthe normal direction of wafer surface at the potion where electrons hitby electron beam irradiation, however, electrons are gradually deflectedin a direction of upward detectors, because electron field for pullingup electrons is generally formed.

While upward acting force by electron field is constant, velocity ofelectrons, when released from an observation object, is not constant,therefore, electrons with high energy, that is, having high velocitywhen released from the object have small degree of deflection byelectric field, due to high having initial velocity. In contrast,electrons with low energy, that is, having low velocity when releasedfrom the object, have large degree of deflection by electric field, dueto having low initial velocity. For example, in the case whereacceleration voltage is about 1 KV when an electron beam is irradiatedto an object, backscattered electron energy is about 1 KeV, while energypeak is 2 to 5 eV when a secondary electron is released from the object.While an electron having low energy with low initial velocity passthrough the hole in the reflecting board 601 regardless of the normaldirection at the point where an electron beam is irradiated, an electronhaving high energy with high initial velocity cannot pass through thehole in the reflecting board 601, and strike on the reflecting board601, or are directly detected by an electron detector 602 or 603. Anelectron striking on the reflecting board 601, release a secondaryelectron again at the reflecting board, which are detected by 602 or603.

As described above, electrons detected by the electron detector 602 or603 become electrons entirely having high energy. Therefore, it becomespossible to form an image generating voltage contrast change based onelectrons detected by the electron detector 111; and an image generatingno voltage contrast change based on electrons detected by the electrondetector 602 or 603.

A system is described for detecting a defect location by obtaining areference image is described so far, however, identification of a defectlocation without obtaining a reference image is required to accomplishhigh throughput at a review apparatus. In particular, in the case wherea defect on a test pattern is reviewed, a defect location is easilydetected without obtaining a reference image, because a test pattern hasperiodicity. In locating a defect by utilization of this periodicity fora defect generating voltage contrast, however, the whole of the voltagecontrast parts cannot be detected sometimes. This case is explained byusing FIG. 7.

In FIG. 7A, a defect image 701 is one including a defect on a testpattern, and voltage contrast phenomenon occurs in a longitudinaldirection, caused by an open defect 702. Generally, in an inspectionapparatus, an inspection system called as a cell comparison system, isknown; wherein a defect is detected by comparing cells themselves apartby integer times of cell part periodicity, at a memory cell part of asemiconductor device mainly; and a microscopic defect can be detectedwith high sensitivity, because of a system to compare cells themselveslocated at a relatively short distance. When this method is applied toan open defect like 702, however, an image to construct with a shift ofinteger times of periode (λ) as a reference image 703 in FIG. 7B, and toinspect a defect image 701 by comparison with the reference image 703, adifference image shown as a difference image 704 in FIG. 7C is obtained,and the whole of voltage contrast cannot be inspected in the case of anopen defect 702.

Because a main judgment standard for voltage contrast in a test patternis large difference of aspect ratio along the pattern at the point ofvoltage contrast difference, by this difference image, it is impossibleto decide whether it is difference occurring by voltage contrast ordifference of actual defect. It is more serious to decide in the case ofa short defect 705 shown in a defect image 701, because a defect partdoes not appear at all in a difference image. To solve this type of aproblem, it is necessary to compare defects based on periodicity of apattern in the direction orthogonal to that where difference of voltagecontrast generates, that is, in the direction orthogonal to theconductive direction of a pattern.

A difference image 706 in FIG. 7D is a difference image, calculated froma reference image (not shown) produced from a defect image 701 in FIG.7A, according to this method, and the defect image 701; and the whole ofdefects including open defects and short defects can be detected. Apattern 707 in FIG. 7D is an open defect part, and a pattern 708 is aghost part generated with reverse polarity to that of the defectsbecause a defect part of a pattern 708 becomes reference part, however,it is possible to distinguish defects from a ghost part, based on thepositional relationship between the pattern 707 and the pattern 708. Apattern 709 is a short defect part, and a pattern 710 is a ghost part ofthe pattern 709.

A defect is not always a small one, but there is the case where thedefect extends across a plurality of patterns. For example, in the casewhere an open defect 801 as shown in FIG. 8A is compared with an imageshifted the position by a small quantity of λ1 relative to the defectsize, the difference image becomes as a difference image 802 in FIG. 8Band the difference image part is separated into two; this suggestspresence of the case where an exact defect location can not beidentified. Even in this case, to make a detection of the whole of adefect part possible, the position to take out a reference image forcalculating a difference image, is set apart as far as possible (λ2);this can solve the problem. Such a setting, however, in an area whichhas no image to take out for comparing at a peripheral part of an imagelike an area 803 surrounded by a heavy line frame in FIG. 8A, makesdetection of a defect impossible. A method to avoid this problem is onefor detecting a defect by reversing polarity only in an area having nocorresponding comparison part like an area 803 surrounded by a heavyline frame in FIG. 8A. For example, in the case where a defect image 801in FIG. 8A is compared with a reference image (not shown) produced byshifting the position by λ2 in the normal part, by comparing aperipheral part of an image like an area 803 surrounded by a heavy lineframe in FIG. 8A, with an area 804 shifted by −nλ2 which is surroundedby a heavy dotted line frame in FIG. 8A, it becomes possible to detect adefect in the whole image even in the case of relatively large λ.

As described above, it is possible to find a defect location, however, adefect is not always present on the surface of a wafer. In FIG. 9showing a cross-sectional view of a test pattern of contact chain, inthe case where a defect is present below a surface 902 of a wafer 900like a defect 901, generally it is impossible to image this by a SEM.Conversely, in the case where a location of a defect found by an opticalinspection apparatus is detected from the image obtained for reviewingby a SEM, the defect can be judged to be present below the surface, whenonly just difference of potential contrast is obtained.

Because, for an inspection of defects, it is quite important to know atwhich process said defects generated, and it is desirable toautomatically judge this point in obtaining a defect image. An algorithmflow for this judgment is shown in FIG. 10. The flow in FIG. 10 is thesame as that in FIG. 5 up to the step for detecting a defect location,however, at the difference calculation step 1001, difference between adefect image and a reference image, is calculated again on an imagewithout generation of difference due to voltage contrast. At the surfaceexposure decision step 1002 based on presence or absence of difference,in the case where difference is generated on an image without generationof difference caused by voltage contrast, it is judged that observationof a defect is possible; and in the case where difference is notextracted, then it is judged that a defect is not exposed on thesurface. The decision result performed at the surface exposure decisionstep 1002 based on presence or absence of difference, is stored in asecondary memory unit 116 at the result storing step 1003.

At the surface exposure decision step 1002 based on presence or absenceof difference, to study the cause of a defect which is not exposed onthe surface, cross-sectional processing may be carried out so that adefect is exposed to a wafer; this process is accomplished by anapparatus called a FIB. A problem to be solved in this process ispositioning for cross-sectional processing by the FIB. Because it isdifficult to place concentrically both an electron optical system forthe FIB processing and an optical system for SEM imaging, the followingmethod may be used: after reviewing by a SEM, cross-sectional processingis conducted by the FIB by transferring a wafer to a different unit; orwith providing both a SEM optical system and a FIB optical system atdifferent positions in the same apparatus, after reviewing by a SEMoptical system, cross-sectional processing is conducted by transferringa wafer to a position where cross-sectional processing is possible bythe FIB.

In the case where a wafer is transferred from a reviewing apparatus to across-sectional processing apparatus, even after alignment is performed,an error of ±1 μm generates, and an error of about ±0.5 μm stillgenerates caused by stage error, even when a SEM optical system and aFIB optical system are provided on the same stage. Incidentally, becausemost of wiring size is generally smaller than or equal to, for example,0.13 μm, unless identifying a defect location again after transferringthe stage, cross-sectional processing cannot be performed due to lack ofpositioning accuracy for cross-sectional processing. Therefore, byirradiating ion beam again by a FIB optical system, and identifying adefect location again, based on an SEM image formed by detecting asecondary electron released from an object, the position forcross-sectional processing is obtained. It should be noted that ion beamirradiated in the FIB could cause contamination.

Therefore, it is desirable that a contaminated area be limited only tothe vicinity of a defect, by identifying a defect location based on theperiodicity of a defect image, without obtaining the reference image asdescribed above. An apparatus configuration in this case, is shown inFIG. 11. 101 to 121 and 601 to 603 are identical with those in FIG. 6.1101 is a FIB optical system, which has both an irradiation system whichirradiates while scanning an ion beam, and a detection system for thesecondary electron released from an object by irradiation of an ionbeam. The imaging object 109 on the wafer 108, after reviewed by a SEMinspection system, is positioned at the area where an image can beobtained by 1101, by transferring of 114 so that cross-sectionalprocessing can be conducted. 1102 is an A/D converter and 1103 is animage memory, where a secondary electron image detected by a FIB opticalsystem is stored. An image stored in 1103 is transferred to imageprocessing means 115 to defect location.

As a technique for detecting a defect location, a method is used fordetecting a defect location from an image detected by a SEM as describedabove. After identifying the defect location, the position forcross-sectional processing is decided so that the defect part becomesexposed or the cross-section becomes exposed. Then the total controlsystem 118 outputs control signals to 1101 for cross-sectionalprocessing, and cross-sectional processing is conducted. FIG. 11 showsan embodiment, wherein a SEM optical system and a FIB optical system forobserving a defect, are provided in the same apparatus, however, theseare not necessarily surface mounted in the same apparatus; it may besurface mounted on different apparatuses.

A method for obtaining a high-magnification image of a defect isdescribed thus far, on the premise of generation of voltage contrastchange by a defect, however, an apparatus is not always a SEM typeinspection apparatus, but there may also be a case of an optical typeinspection apparatus. In this case, when generation of voltage contrastchange is observed by a SEM type review apparatus, it can be confirmedthat a defect detected by an optical type inspection apparatus is anelectrical fault; and when there is no generation of voltage contrastchange, it can be confirmed not an electrical fault.

In a general semiconductor device circuit, even when an electrical faultgenerates, voltage contrast change does not always generate and thusreliability of such decision is low, however, in the case where a testpattern is formed, wherein voltage contrast change generates easily,such decision can be made with high reliability.

A schematic diagram for this case is shown in FIG. 12. 1201 is anoptical type inspection apparatus, wherein inspection is performed oneto several times during a pattern formation to output defectcoordinates. 1202 is a group of manufacturing apparatuses. Thoseobtained by merging of each inspection results are used as defectcoordinates. At the stage after a pattern including etching finished isformed, by reviewing a defect by 1203 based on the merged defectcoordinates, it is decided whether or not a defect is accompanied byvoltage contrast change. A defect with voltage contrast change isclassified as an electrical defect. Based on the decision of whether ornot output defect by each inspection apparatus is an electrical defect,judgment is made in which manufacturing apparatus many electricaldefects are found, by a yield diagnosis system 1204; thus, it ispossible to find a manufacturing apparatus having an impact on yield. Asdescribed above, the present reviewing can achieve both high-speedinspection by an optical inspection apparatus comparing with a SEM type,and yield impact evaluation by observation of voltage contrast change byusing a SEM type review apparatus.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A method for reviewing defects, the method comprising the followingsteps: using positional information of defects on a test sample detectedby a defect detecting apparatus to irradiate and scan an electron beamfocused on an area including said defects in said test sample; detectingsecondary charged particles generated from said test sample, byirradiating and scanning the electron beam, under a first detectioncondition to obtain a first inspection image of the area including saiddefects; detecting the secondary charged particles generated from saidtest sample, by irradiating and scanning the electron beam, under asecond detection condition to obtain a second inspection image of thearea including said defects; comparing said first inspection image witha first reference image to obtain a first binarized image by voltagecontrast; comparing said second inspection image with a second referenceimage to obtain a second binarized image by voltage contrast; judgingwhether said defects are open defects or short defects from said firstbinarized image; identifying a generating location of the open defectsby processing said first binarized image in the case where the defectsare judged as open defects as a result of the judgment; and identifyinga generating location of short defects by processing said secondbinarized image in the case where the defects are judged as shortdefects as a result of the judgment.
 2. A method for reviewing defectsas recited in claim 1, wherein said first detecting condition is acondition detecting the secondary charged particles generated from saidtest sample without cutting the secondary charged particles having lowenergy, and said second detecting condition is a condition detectingsecondary charged particles after cutting the secondary chargedparticles having low energy from the secondary charged particlesgenerated from said test sample.
 3. A method for reviewing defects asrecited in claim 2, wherein in the case of judging whether said defectsare open defects or short defects from said first binarized image, in abinarized image obtained by comparing an image obtained without cuttingsaid secondary charged particles having low energy with a referenceimage, when the difference from said reference image is minus, saiddefects are judged as open defects, and when said difference from thereference image is plus, said defects are judged as short defects.
 4. Amethod for reviewing defects as recited in claim 1, wherein the firstreference image is an image obtained, under said first detectingcondition, by detecting the secondary charged particles generated fromsaid test sample by irradiating and scanning the electron beam focusedon the area including said defects on said test sample, and said secondreference image is an image obtained, under said second detectingcondition, by detecting the secondary charged particles generated fromsaid test sample by irradiating and scanning the electron beam focusedon the area including said defects on said test sample.
 5. A method forreviewing defects as recited in claim 1, wherein said first referenceimage is an image produced by composing of said first inspection image,and said second reference image is an image produced by composing of thesecond said inspection image.
 6. A method for reviewing defects asrecited in claim 1, wherein irradiating and scanning the electron beamfocused on the area including said defects on said test sample isconducted twice, to obtain said first inspection image by irradiatingand scanning said electron beam for the first time, and to obtain saidsecond inspection image by irradiating and scanning said electron beamfor the second time.
 7. A method for reviewing defects as recited inclaim 1, wherein said first inspection image and said second image areobtained simultaneously by irradiating and scanning the electron beamfocused on the area including said defects on said test sample.
 8. Anapparatus of reviewing defects, the apparatus comprising the followingconfiguration: a defect location information inputting means whichinputs location information of defects on a test sample, detected by adefect inspection apparatus; an electron beam irradiating and scanningmeans which irradiates and scans the electron beam focused on the areaincluding said defects, by using defect location information on saidtest sample having input in the defect location information inputtingmeans; an image obtaining means, which obtains a first inspection imageof the area including said defects under the first detecting condition,and obtains a second inspection image of the area including said defectsunder the second detecting condition, by detecting the secondary chargedparticles generated from said test sample by irradiating and scanningthe electron beam with the electron beam irradiating and scanning means;an image processing means, which identifies a generating location ofopen defects or short defects by processing said first inspection imageand said second inspection image, obtained by said image obtainingmeans; and an output means which outputs results of processing performedby the image processing means; wherein said image processing meanscompares said first inspection image with a first reference image toobtain a first binarized image by voltage contrast; compares said secondinspection image with a second reference image to obtain a secondbinarized image by voltage contrast; judges whether said defects areopen defects or short defects from said first binarized image toidentify a generating location of the open defects by processing saidfirst binarized image in the case where the defects are judged as opendefects as a result of the judgment, and to identify the generatinglocation of short defects by processing said second binarized image inthe case where the defects are judged as short defects as the result ofthe judgment.
 9. An apparatus of reviewing defects as recited in claim8, wherein said image obtaining means has an energy filter and adetector, which are capable of switching potential, and in the case ofdetection under said first detecting condition, detects, by saiddetector, the secondary charged particles generated from said testsample, while keeping a state of not applying potential to potential ofsaid energy filter, without cutting the secondary charged particles withlow energy, and in the case of detection under said second detectingcondition, detects, by said detector, the secondary charged particlesgenerated from said test sample, while keeping the state of applyingpotential on said energy filter, with cutting the secondary chargedparticles with low energy.
 10. An apparatus of reviewing defects asrecited in claim 8, wherein said image obtaining means has a firstdetector which detects the secondary charged particles generated fromsaid test sample, with the secondary charged particles having lowenergy; a reflecting board which releases a secondary electron byirradiation of the secondary charged particles having relatively highenergy, generated from said test sample; and a second detector whichdetects the secondary electron generated from the reflecting board. 11.An apparatus for reviewing defects as recited in claim 9, wherein, insaid image processing means, for judging whether said defects are opendefects or short defects, from the first binarized image, in thebinarized image obtained by comparing an image obtained by detecting thesecondary charged particles generated from the test sample, by the imageobtaining means, without cutting the secondary charged particles havinglow energy, with a reference image, in the case where difference withthe reference image is minus, said defects are judged as open defects;while in the case where difference with the reference image is plus,said defects are judged as short defects.
 12. An apparatus for reviewingdefects as recited in claim 10, wherein, in said image processing means,for judging whether said defects are open defects or short defects, fromsaid first binarized image, in the binarized image obtained by comparingan image obtained from detecting the secondary charged particlesgenerated from the test sample, by said image obtaining means, withoutcutting the secondary charged particles having low energy, with thereference image, in the case where difference with said reference imageis minus, the defects are judged as open defects; while in the casewhere difference with the reference image is plus, the defects arejudged as short defects.
 13. An apparatus for reviewing defects asrecited in claim 8, wherein, in said image processing means, the firstreference image which is compared with the first inspection image, is animage obtained by detecting the secondary charged particles generatedfrom said test sample by irradiating and scanning the electron beamfocused on the area including said defects on said test sample, undersaid first detecting condition; said second reference image which iscompared with the second inspection image, is an image obtained bydetecting the secondary charged particles generated from said testsample by irradiating and scanning the electron beam focused on the areaincluding said defects on said test sample, under said second detectingcondition.
 14. An apparatus for reviewing defects as recited in claim 8,wherein, in said image processing means, said first reference imagewhich is compared with the first inspection image, is an image producedby constructing the first inspection image, and in the image processingmeans, the second reference image which is compared to the secondinspection image is an image produced by constructing the secondinspection image.